[BUG] Custom headers in `notmuch-message-headers` are broken

[Leo Gaspard]

(sorry for sending twice, forgot to Cc the list at first)

I just have been hit by this exact same issue, also for X-GitHub-Sender,
during my switch to notmuch and notmuch-mode.

>> 3) We should think carefully about whether we want to blindly send
>>    certain large headers like "Received". Some people use notmuch via
>>    ssh or equivalent, and it might (dunno) be a concern.
>
> I'd prefer `notmuch show` to dump everything by default and have an
> option like `--headers` to limit those. I.e. to get current behavior
> you'd just dump comma-separated `notmuch-message-headers` into that
> option in `notmuch.el` and be happy.

Potentially keep the `--headers` option as you propose and default to
the current behaviour? This way everything is retro-compatible, messages
still look nice when manually shown via `notmuch show` by default, but
tools that make use of the additional headers can specify which headers
they use.

And then dumping comma-separated `notmuch-message-headers` into this
option becomes the additional feature for `notmuch.el` we're all hoping
for :)